Studying the Irradiation Effect on the Thermal Creep Properties of Zircalloy-2 via Indentation Creep Test Method

Abstract

This dissertation studied the irradiation effect on the thermal creep properties of reactor-used Zirconium alloy via proton irradiation and indentation creep test method. The general introduction in Chapter 1 discussed the background and motivation as well as the planned works of this dissertation. Chapter 2 presents the literature review related to this study. In Chapter 3, we explored the method to determine the creep parameters based on indentation creep test results, in order to understand the creep deformation mechanisms. An iterative FEM simulation method was performed to get the best fit of the experimental indentation creep test data. A strain hardening creep model is developed to predict the low temperature creep behaviour of Zircaloy-2. The creep mechanism of Zircaloy-2 and irradiation effect is discussed according to the creep parameters including stress exponent (n), activation volume (V*) and activation energy (Q) that calculated based on indentation creep data. To better understand the indentation creep mechanism and irradiation effect, in Chapter 4, TEM through-thickness lift-outs were made using FIB under the indents to characterize the deformation microstructure of the plastic zone. The temperature and irradiation effect on the creep mechanism are then discussed. In Chapter 5, the source mechanism of c+a dislocation was studied by examining the deformation microstructure at the edge of the plastic zone. Chapter 6 is the general conclusion of this dissertation. At temperature below 200 °C, dislocation glide by overcoming Peierls stress could be the predominant creep mechanism. The plastic zone expands gradually by gliding of dislocations into the undeformed area. At temperatures above 200 °C, annihilation of dislocation by pipe diffusion-controlled dislocation climbing could be the dominant creep mechanism. Irradiation induced loops impede the gliding of a-type dislocations and annihilation of dislocations, higher activation energy is required for the creep deformation of irradiated sample. Larger plastic zones are required for deformation of irradiated samples due to the localized deformation behavior. Small loops form at the edge of plastic zone, dislocation nucleation could be the creep mechanism under low stress. dislocations are proved can nucleate inside the grain in the absence of a-type dislocation activity.